全文获取类型
收费全文 | 17690篇 |
免费 | 762篇 |
国内免费 | 88篇 |
专业分类
化学 | 10451篇 |
晶体学 | 240篇 |
力学 | 490篇 |
综合类 | 1篇 |
数学 | 1163篇 |
物理学 | 3747篇 |
无线电 | 2448篇 |
出版年
2024年 | 86篇 |
2023年 | 243篇 |
2022年 | 597篇 |
2021年 | 640篇 |
2020年 | 682篇 |
2019年 | 694篇 |
2018年 | 661篇 |
2017年 | 584篇 |
2016年 | 828篇 |
2015年 | 535篇 |
2014年 | 894篇 |
2013年 | 1499篇 |
2012年 | 1219篇 |
2011年 | 1265篇 |
2010年 | 848篇 |
2009年 | 679篇 |
2008年 | 810篇 |
2007年 | 804篇 |
2006年 | 660篇 |
2005年 | 576篇 |
2004年 | 452篇 |
2003年 | 388篇 |
2002年 | 302篇 |
2001年 | 196篇 |
2000年 | 168篇 |
1999年 | 129篇 |
1998年 | 96篇 |
1997年 | 129篇 |
1996年 | 135篇 |
1995年 | 104篇 |
1994年 | 104篇 |
1993年 | 126篇 |
1992年 | 116篇 |
1991年 | 94篇 |
1990年 | 89篇 |
1989年 | 82篇 |
1988年 | 63篇 |
1987年 | 58篇 |
1986年 | 55篇 |
1985年 | 81篇 |
1984年 | 73篇 |
1983年 | 63篇 |
1982年 | 54篇 |
1981年 | 52篇 |
1980年 | 60篇 |
1979年 | 66篇 |
1978年 | 61篇 |
1977年 | 61篇 |
1976年 | 46篇 |
1975年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
D. M. Bhardwaj D. C. Jain S. Dalela Ravi Kumar N. L. Saini K. B. Garg 《Physica B: Condensed Matter》2004,350(4):366-374
We have made the XAFS measurements at the Cr–K-edge on natural Indian ruby single crystals (corundum) and its two irradiated samples with fluence 1×1012 Ni6+ and 5×1012 Ni6+ ions/cm2. Irradiated samples show interesting changes in their physical appearance. XANES measurements show progressive decrease in Δoct value on increase of Ni fluence in irradiated samples. EXAFS measurements on these samples show decrease in Cr–O distance on increase of Ni fluence. Lowering of Δoct value is correlated with the increase of Cr–O distance. 相似文献
92.
93.
We have calculated total and differential cross-sections for 1s →ns (n = 2, 3, 4) electron impact excitation of hydrogen and hydrogenic ions at various energies in Coulomb-projected Born approximation.
Distortion due to static interactions, target polarization and exchange effects has been incorporated in the initial channel.
The present calculations have been compared with other theoretical and experimental results. 相似文献
94.
We report a new structure, called the shielded ohmic contact (ShOC) rectifier which utilizes trenches filled with a high-barrier metal to shield an Ohmic contact during the reverse bias. When the device is forward biased, the ohmic contact conducts with a low forward drop. However, when reverse biased, the Ohmic contact is completely shielded by the high-barrier Schottky contact resulting in a low reverse leakage current. Two dimensional numerical simulation is used to evaluate and explain the superior performance of the proposed ShOC rectifier. 相似文献
95.
96.
Homopolymerization of methyl methacrylate (MMA) was carried out in the presence of triphenylstibonium 1,2,3,4-tetraphenyl-cyclopentadienylide
as an initiator in dioxane at 65°C±0·l°C. The system follows non-ideal radical kinetics (R
p
∝ [M]1·4 [I]0·44
@#@) due to primary radical termination as well as degradative chain-transfer reaction. The overall activation energy and average
value ofk
2
p
/k
t
were 64 kJmol−1 and 0.173 × 10−3 1 mol−1 s−1 respectively 相似文献
97.
A short and efficient enantioselective synthesis of sulfobacin A has been achieved using the Sharpless asymmetric dihydroxylation and the regiospecific nucleophilic opening of a cyclic sulfate as the key steps. 相似文献
98.
Dalip Kumar 《Tetrahedron letters》2004,45(14):2887-2889
Standard procedures for elaborating a quinone methide precursor for conjugation to a DNA ligand was not compatible with the presence of a vinyl group. Instead, an acrylate linker was attached by Heck coupling subsequent to o-substitution of the phenolic precursor. This transformation required protection of the phenolic group and use of ethyl acrylate rather than acrylic acid. The presence of the vinyl group also rendered the quinone methide precursor more labile to alkaline conditions than its equivalent saturated derivative and required mild conditions for coupling to the pyrrole-imidazole polyamide. 相似文献
99.
An age-dependent branching process where disasters occur as a renewal process leading to annihilation or survival ofall the cells, is considered. For such a process, the total mean sojourn time of all the cells in the system is analysed using the regeneration point technique. The mean number of cells which die in timet and its asymptotic behaviour are discussed. When the disasters arrival as a Poisson process and the lifetime of the cells follows exponential distribution, elegant inter-relationships are found among the means of (i) the total number of cells which die in timet (ii) the total sojourn time of all cells in the system upto timet and (iii) the number of living cells at timet. Some of the existing results are deduced as special cases for related processes. 相似文献
100.
The effect of solar features on geospheric conditions leading to geomagnetic storms (GMSs) with planetary index,A
P ≥ 20 and the range of horizontal component of the Earth’s magnetic fieldH such that 250γ <H < 400γ has been investigated using interplanetary magnetic field (IMF), solar wind plasma (SWP) and solar geophysical data
(SGD) during the period 1978–99. Statistically, it is observed that maximum number of GMSs have occurred during the maximum
solar activity years of 21st and 22nd solar cycles. A peculiar result has been observed during the years 1982, 1994 when sunspot
numbers (SSNs) decrease very rapidly while numbers of GMSs increase. No distinct association between yearly occurrence of
disturbed days and SSNs is observed. Maximum number of disturbed days have occurred during spring and rainy seasons showing
a seasonal variation of disturbed days. No significant correlation between magnitude (intensity) of GMSs and importance ofH
α
, X-ray solar flares has been observed. Maximum number of GMSs is associated with solar flares of lower importance, i.e.,
SF during the period 1978-93.H
α
, X-ray solar flares occurred within lower helio-latitudes, i.e., (0–30)°N to (0–30)°S are associated with GMSs. NoH
α
, X-ray solar flares have occurred beyond 40°N or 40°S in association with GMSs. In helio-latitude range (10–40)°N to (10–40)°S,
the 89.5% concentration of active prominences and disappearing filaments (APDFs) are associated with GMSs. Maximum number
of GMSs are associated with solar flares. Coronal mass ejections (CMEs) are related with eruptive prominences, solar flares,
type IV radio burst and they occur at low helio-latitude. It is observed that CMEs related GMS events are not always associated
with high speed solar wind streams (HSSWSs). In many individual events, the travel time between the explosion on the Sun and
maximum activity lies between 58 and 118 h causing GMSs at the Earth. 相似文献